A user equipment (UE) is required to meet overall regional Specific Absorption Rate (SAR) transmit power requirements regardless of whether single or dual/multiple transmission using different radio access technologies (RATs) is supported. Regional SAR requirements impose restrictions on the maximum RF power density close to the UE. In the case of dual/multiple transmission UEs, the implications of this restriction on the transmit power are dependent on various factors such as the number and location of the antennas (whether two antennas are used, the distance between them, etc.) and the form factor of the UE. In some situations this can place additional limits on the transmission powers of one or more air interfaces when they are operating concurrently.
Further, when there is multiple transmission on different RATs, inter-modulation products may arise that cause interference on neighboring carriers. This can likewise place constraints on the transmission powers on one or more RATs when they are operating concurrently.
A problem that arises from such power reductions is that there is no standard mechanism of informing the network (scheduler) of the reduction of transmission power due to the dual/multiple transmission. Not knowing up-to-date information of available transmission power at the UE, the network may schedule the UE for higher transmission power which the UE may fail to deliver, hence impacting system performance. In addition, there is a possibility for mismatch between network and the UE on power control, which also leads to inaccuracy in channel estimation at the network. This also results in scheduling inaccuracy, hence reduction of system performance.
A further problem is that there is no defined way to specify the allowed reductions in transmission power, with the result that UEs may choose to reduce their transmission power by arbitrary amounts, resulting in unpredictable performance.
There is no standard compliant mechanism for providing transmission power information to the network (scheduler) taking into account the power reduction due to the dual/multiple transmission.
A conventional mechanism to trigger the transmission of power headroom reporting (PHR) to the network exists. However, the PHR only provides information about the difference between nominal UE maximum transmit power, PCMAX, and the estimated power of UL transmission. Therefore, PHR does not convey information to the network of the cause of power reduction or the amount of power reduction due to, for example, dual/multiple transmission. Maximum output power of the UE, PCMAX, is defined as a value range in the current LTE specification (Rel-8/9 LTE). The UE sets PCMAX to a value within the value range. The value range depends on UE power class, the maximum transmit power signalled by the network, PEMAX, maximum power reduction permitted based on modulation and transmit bandwidth configuration, MPR, additional maximum power reduction permitted based on additional requirements for uplink transmission to the network (e.g., eNodeB), A-MPR, and a reduction term dependent on the transmission bandwidth, ΔTC. PCMAX, the value range and the above discussed parameters are defined in TS 36.101 V10.0.0.
The UE is allowed to set its maximum output power PCMAX. The configured maximum output power PCMAX is set within the following bounds:PCMAX—L≦PCMAX≦PCMAX—H  (1)wherePCMAX—L=MIN{PEMAX−ΔTC, PPowerClass−MPR−A-MPR−ΔTC}  (2)PCMAX—H=MIN{PEMAX, PPowerClass}  (3)                PEMAX is the value broadcast by the network in system information. PEMAX takes into account the inter-cell interference co-ordination.        PPowerClass is the maximum UE power specified in the standard and available to the UE from tables stored in the UE.        MPR and A-MPR are specified in the standard and available from tables stored in the UE. MPR depends on the modulation and transmit bandwidth configuration, while A-MPR depends on additional requirements for uplink transmission to the network (e.g., eNodeB). For example A-MPR depends on frequency band and resource region allocated for the UL transmission to the eNodeB.        ΔTC is defined in the standard and depends on the frequency band for uplink transmission.        
Radio resource control (RRC) controls power headroom reporting by configuring two timers periodicPHR-Timer and prohibitPHR-Timer and by signalling a threshold value dl-PathlossChange. The PHR is triggered if the following criterion is met:                prohibitPHR-Timer expires or has expired and the path loss has changed more than dl-PathlossChange dB since the last transmission of a PHR when UE has up link (UL) resources for new transmission;        periodicPHR-Timer expires;        upon configuration or reconfiguration of the power headroom reporting functionality by upper layers, which is not used to disable the function.        